1. Introduction to GPS Data NMEA & RTCM
Donald Choi, ALS/G2

2. Common GPS Data Format RINEX NMEA RTCM
Combine data from different manufacturer’s GPS receivers
For static data processing and archive
NMEA
Transmission of data between GPS receiver and other devices (e.g. GPS antenna PDA with ArcPad software)
For real time positioning
RTCM
Transmission of data between GPS receivers (e.g Base  Rover)
Binary file (more compact but difficult to understand)
For real time DGPS/RTK corrections

3. Common GPS Data Format
NMEA Data

4. Introduction Background Message Format and Sentence structures
Common NMEA sentence
Other GPS-related NMEA sentence
References

5. Flow of NMEA data between devices
Real-time positioning using the NMEA data
NMEA data transfer from GPS antenna to PDA with ArcPAD software

6. NMEA data used in Network RTK
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
RTCM SC-104 Ver. 2.1
Fix the Ambiguity
Ref. Stn X,Y,Z
Pseudo-range + Corrn.
Carrier Phase + Corrn.
cm position accuracy
Data Centre
NMEA 0183
Real Time Position of Rover

7. What is NMEA data?
Developed by National Marine Electronics Association
Standard for interfacing marine electronic devices (GPS, , Compass, Echosounder…)
Standard specifies the electrical signal requirements, data transmission protocol (i.e. agreed format for transmitting data between two devices), message formats
Most GPS receivers understand the most popular NMEA format  NMEA 0183

8. What is NMEA data? ASCII file Numerous sentence types
Easily readable (even by people)
Less compact than binary
Numerous sentence types
Not all related GPS application
Some popular GPS-related sentences are described in this presentation

9. NMEA settings Baud rate: 4800 (4800 characters in one second )
Data bits: 8 (Bit 7 set to 0)
Stop bits: 1 or 2
Parity: none
The “send” device and “receive” device should have same setting so as to transfer the NMEA data

10. Standard Message Format
$aaaaa, df1,df2……… [Carriage Return][Line Feed]
Each message start with $
Five characters after $ are address field
Multiple data fields delimited by commas
Check sum (optional): a 2-character field (0-9,A-F) to increase data integrity
(*) character placed after the last data field

11. Sentence structure There are three sentence structures
Talker: output from talker (e.g. GPS receiver) to other devices (PAD)
Query: means for listener (e.g. notebook) to request specified talker sentence from talker (GPS receiver)
Proprietary: means for manufactures to use non-standard sentences for special purpose

12. Talker sentence Format: $ttsss, df1, df2…[CR][LF]
tt: defines device (for gps receivers the prefix is GP)
sss: sentence type
Example
$GPGGA,031956, ,N, ,E,1,04,3,9,005.9,M,-001.3,M,,*51
The talker sends the GPS data in GGA sentence format.

13. Query sentence Format: $ttllQ, sss, [CR][LF] Example
CCGPQ,GGA, [CR][LF]
A computer (CC) is requesting from GPS device (GP) the GGA sentence

14. Proprietary sentence Format: $PmmmA,df1,df2,….[CR][LF] Example
“P” indictes it is a proprietary message
“mmm” define as the manufacture
The fifth character is a letter (A-Z) defines the specific message type
Example
$PLEIS,AHT,0,1.90,0*34
“LEI" is Leica manufacturer
“S” for set command
The message set the antenna height at 1.9m in the configuration of the internal sensor setting.

15. Common NMEA-0183 Sentence Sentence Description
$GPGGA: Global positioning system fixed data
$GPGLL: Geographic position - latitude / longitude
$GPGSA: GNSS DOP and active satellites
$GPGSV: GNSS satellites in view
$GPRMC: Recommended minimum specific GPS data
$GPVTG: Course over ground and ground speed
To extract information related to surveyed position
Record at least one of the 3 sentences: $GPGGA, $GPGLL, $GPRMC

16. GGA Sentence Format Field Example Comments Sentence ID $GPGGA
$GPGGA, , ,S, ,E,1,04,24.4,19.7,M,,,,0000*1F
Field Example Comments
Sentence ID $GPGGA
UTC Time hhmmss.sss
Latitude ddmm.mmmm
N/S Indicator S N = North, S = South
Longitude dddmm.mmmm
E/W Indicator E E = East, W = West
Position Fix = Invalid, 1 = Valid SPS,
2 = Valid DGPS, 3 = Valid PPS
Satellites Used Satellites being used (0-12)
HDOP Horizontal dilution of precision
Altitude Altitude (WGS-84 ellipsoid)
Altitude Units M M= Meters
Geoid Separation  Geoid separation (WGS-84 ellipsoid)
Seperation Units  M= Meters
Time since DGPS   in seconds
DGPS Station ID  
Checksum *1F  always begin with * 

17. GGA Sentence Format (2) Fix quality: 0 = invalid 1 = GPS fix (SPS)
$GPGGA, , ,S, ,E,1,04,24.4,19.7,M,,,,0000*1F
Fix quality:
0 = invalid
1 = GPS fix (SPS)
2 = DGPS fix
3 = PPS fix
4 = Real Time Kinematic
5 = Float RTK

18. GLL Sentence Format $GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D
Field Example Comments
ID $GPGLL
Latitude ddmm.mmmm
N/S Indicator S N = North, S = South
Longitude dddmm.mmmm
E/W Indicator E E = East, W = West
UTC Time hhmmss.sss
Status A A = Valid, V = Invalid
Checksum *2D 

19. GSA Sentence Format $GPGSA,A,3,01,20,19,13,,,,,,,,,40.4,24.4,32.2*0A
Field Example Comments
Sentence ID $GPGSA 
Mode A A = Auto 2D/3D
M = Forced 2D/3D
Mode = No fix, 2 = 2D, 3 = 3D
Satellite used Satellite used on channel 1 .
Satellite used Satellite used on channel 12
PDOP Position dilution of precision
HDOP Horizontal dilution of precision
VDOP Vertical dilution of precision
Checksum *0A

20. GSV Sentence Format
$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70
Field Example Comments
Sentence ID $GPGSV 
No. of messages No. of messages in complete (1-3)
Sequence no Sequence no. of this entry (1-3)
Satellites in view 10
Satellite ID Range is 1-32
Elevation Elevation in degrees
Azimuth Azimuth in degrees
SNR Signal to noise ratio dBHZ (0-99)
Satellite ID Range is 1-32
Elevation Elevation in degrees
Azimuth Azimuth in degrees .
Checksum *70

21. RMC Sentence Format
$GPRMC, ,A, ,S, ,E,0.00,89.68,211200,,*25
Field Example Comments
Sentence ID $GPRMC
UTC Time hhmmss.sss
Status A A = Valid, V = Invalid
Latitude ddmm.mmmm
N/S Indicator S N = North, S = South
Longitude dddmm.mmmm
E/W Indicator E E = East, W = West
Speed over ground Knots
Course over ground Degrees
UTC Date DDMMYY
Magnetic variation  Degrees
Magnetic variation   E = East, W = West
Checksum *25  

22. VTG Sentence Format $GPVTG,89.68,T,,M,0.00,N,0.0,K*5F
Field Example Comments
Sentence ID $GPVTG 
Course Course in degrees
Reference T = True heading
Course  Course in degrees
Reference M = Magnetic heading
Speed Horizontal speed
Units N N = Knots
Units K K = KM/h
Checksum *5F  

23. NMEA-0183 data file

24. Other GPS NMEA sentences
Sentence Description
$GPALM: GPS almanac data
$GPGRS: GPS range residuals
$GPGST: GPS pseudorange noise statistics
$GPMSS: Beacon receiver status
$GPZDA: UTC and local date/time data

25. References Common NMEA Setence types NMEA data
NMEA data
The National Marine Electronics Association

26. RTCM correction
Common GPS Data Format

27. Outline Background RTCM Data Format RTCM Message Types
Transmission method of RTCM data
RTCM data used in Network-RTK

28. RTCM correction used in Network RTK
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
RTCM SC-104 Ver. 2.1
Fix the Ambiguity
Ref. Stn X,Y,Z
Pseudo-range + Corrn.
Carrier Phase + Corrn.
cm position accuracy
Data Centre
NMEA 0183
Real Time Position of Rover

29. RTCM correction used in DGPS service
Kau Yi Chau DGPS Sevices
Radio transmission
The corrections are formatted to RTCM 2.0 and broadcast via Radio antenna
RTCM SC-104 V.2.0

30. What is RTCM SC-104 data?
Developed by Radio Technical Commission for Maritime Services (RTCM)
RTCM organization consists of various special committees to develop international standards for maritime radionavigation and radiocommunication systems
RTCM Special Committee (SC) 104 is the one concerned Differential Global Navigation Satellite System (DGNSS)
RTCM data format related to GPS applications is called RTCM SC-104

31. What is RTCM SC-104 data?
Industry standard for Real time differential data
Continuous and compact binary datagood for real-time processing
Has been different versions
RTCM2.0
RTCM2.1
RTCM 2.2
RTCM 2.3
RTCM 3.0

32. RTCM Data Format
A data record contains several message types for various contents
Message 1, Message2…. Message N
Each message comprises header and body
Header: message type, time, length of message,
Body: data for every data type
Each data record may not include every message type
Some send every second (e.g. code corrections)
Others are sent at interval (e.g. base station coordinates)

33. Features of various RTCM versions
for DGPS applications only (message 1,9) and no RTK data
RTCM 2.1
retain the full content of RTCM 2.0
add new messages for carrier phase data and RTK corrections
RTCM 2.2
contain GLONASS data and associated information
store in newly added messages 31-36
RTCM 2.3
antenna types (message 23)
ARP information (message 24)
RTCM 3.0
RTCM 2.3 requires 4800 bps to broadcast dual-frequency code and carrier-phase observation corrections of 12 satellites. The information content is send with 1800 bps in RTCM 3.0
accommodate for new GNSS systems that are under development (e.g. Galileo)

34. RTCM Message Types

35. RTCM Message Types New messages added in RTCM2.1 New messages

36. RTCM Message Types
New messages
added in RTCM2.3

37. RTCM 3.0 Messages Type

38. RTCM Raw Data

39. RTCM Message Type 1

40. RTCM Message Type 2

41. RTCM Message Type 3+22

42. RTCM Message 20

43. RTCM Message Type 21

44. Transmission method of Real-time RTCM data
Global System for Mobile Communication (GSM)
GSM modem and SIM card with service available
Charged by connection time
General Packet Radio Service (GPRS)
An upgrade to GSM
Bridging the mobile network to IP network
Charged by the amount of download data but not the connection time
Cheaper than GSM

45. Latency of DGPS and RTK correction

46. References DGPS Data Formats 2.0
RTCM recommended standards for differential NAVSTAR GPS service
RTCM Official Website
Reference Station Network Information Distribution

47. The End Thank you!